1. Field of the Invention
The present invention relates to an image distribution system which distributes image data from a server via a network, a client terminal, and a control method thereof.
2. Description of the Related Art
In recent years, an image distribution system which distributes captured image data from a server to a client terminal has prevailed. In such a system, the server comprises, e.g., a network camera, and the client terminal comprises a computer. Such an image distribution system is used especially for the purpose of surveillance of specific places by utilizing a feature of display images from cameras arranged at a plurality of remote places.
Upon making surveillance based on images, images from a large number of servers must be simultaneously displayed on a display of the client terminal. Hence, the images from the servers are respectively displayed in reduced windows on the display. If a particular image is selected, the window of that image can be displayed in an enlarged size. The size of the window is changed by enlarging or reducing the window size of an application that displays the image. More specifically, a user often enlarges or reduces the window size to a desired size by dragging a window frame displayed by the application with a mouse.
On the other hand, each server transmits captured image data at a resolution which is set in advance to the client terminal. A server which can change this resolution in response to a request from the client terminal is commercially available. For example, a watcher can select and set one of resolutions of 640×480 (pixels) and 320×240 (pixels) in the server, and can receive and display image data which is transmitted at the set resolution. The resolution in the server can be easily changed by halving number of pixels to be sampled if that resolution is ½ the maximum resolution of the image data (see Japanese Patent Laid-Open No. 11-196379).
Therefore, as the resolution provided by the server, a standard resolution, for example, a VGA size (640×480 (pixels)), of a computer or image media is set as a maximum resolution. Then, the server normally provides a QVGA size (320×240 (pixels)) ½ the VGA size, and a QQVGA size (160×120 (pixels)) ½ the QVGA size. However, the following configuration may also be adopted. That is, an image is captured using a high-resolution CCD in advance and an image area according to an arbitrary resolution is transmitted, or when resolution conversion is made using a high-speed image processing LSI, the resolution of an image to be transmitted from the server can be changed to an arbitrary resolution other than the prescribed values.
Upon displaying image data distributed from the server on the client terminal, when the client terminal changes the image window to an arbitrary size, it executes resizing processing for changing the rendering size. For example, upon displaying an image which is distributed from the server and has a resolution of 640×480 (pixels), the size of the window that displays the image is reduced at a client terminal. In this case, the client terminal executes the resizing processing from the resolution of 640×480 (pixels) to that of, e.g., 500×300 (pixels) in accordance with the size of the reduced window.
In such resizing processing at the client terminal, since the interpolation processing and re-sampling processing between neighboring pixels are executed upon image display, a heavy load is imposed on the processing at the client terminal. Especially, when the client terminal connects a large number of servers and displays a large number of images from the servers, and when it simultaneously applies resizing processing to these pictures, the load on the client terminal increases significantly. Such load increase results in a drop in frame rate upon displaying an image, and a drop in processing speed of the computer. That is, the load of the resizing processing according to various image qualities required by the watcher or the operation states of various supervising apparatuses used by the watcher cannot be reduced.
Enlarged-scale display by means of the interpolation processing based on neighboring pixels in the client terminal cannot obtain an optimal image quality according to the window size. For example, when an image window that displays image data of 160×120 (pixels) is enlarged to 640×480 (pixels), since data before resize is enlarged four times in the vertical and horizontal directions, the image quality deteriorates.